DIY solar powered alarm clock
 

So, I'm in the market for a new alarm clock. My current one is about 10 years old and it has started making an annoying electrical buzzing noise like a bad florescent light. I originally thought it would be cool to get a solar rechargeable alarm clock! Clocks don't use much power and it would be nice to have one less thing drawing 1-3W continuously.

I went on amazon and started searching. I only found a few solar powered alarm clocks and none of them really met my needs (which are actually quite simple). I wanted an alarm clock that I can read at night, none of that pushing a button to light it up stuff since the alarm clock is slightly out of reach. None of the solar powered alarm clocks had this feature, arguably to save on battery life.

What is a guy to do if he can't buy what he wants? DIY of course! :thumbup:

So, I started looking for battery powered alarm clocks of which they are many. You can always add a solar panel to something with batteries. One feature I found while searching is an automatic back lighting feature. They have a light sensor on the clock and when it gets dark in the room it turns the back light on. This saves power during the day and extends battery life.

All I need to do is select a clock and then pick out a solar panel and build a small charge controller for it. Sounds like a fun little project, and I get to get my hands a little dirty with some solar stuff.

Here are a couple of the clocks I am looking at:

Amazon.com: Westclox LCD Alarm Clock 70045: Home & Kitchen

Amazon.com: Elgin 2-Inch LCD Multifunction Alarm with Smartlite: Home & Kitchen


Right now I'm leaning toward the 2nd one because it uses 3 AA batteries vs 3 AAA batteries and I wouldn't mind having a bit more battery life. However, even with the AAAs, since we're hooking up a solar panel, it should hopefully be a non-issue.

I also did some quick searching for a DIY charge controller for nimh batteries and found this:

Float Charging NiMH Cells [227] | General Electronics | Electronics

It uses a 12V supply which is a bit high. I was thinking a 5-6V solar panel would be fine. I haven't looked at it quite enough to see if it will just work with a lower voltage panel. Much more searching to do.

Sounds like an entertaining project. First thought is to canibalize one (maybe two) solar path lights for the whole solar kit. I would think that would be enough. My wife has a travel alarm that only needs one AAA every two or three years, maybe longer.

Interesting. Subscribed.

I'm partial to wind-up clocks, but they don't usually have backlights! :D

So, I've been doing some reading on nimh chemistry (which I do know some stuff about, but not charging specifically). I found this specific site quite informative:

How to charge Nickel Metal Hydride Batteries.


So, in order to keep the system super simple and easy all I need is to provide approximately 1.41V per cell and keep the current below C/10.

Lets start with the C/10 part. C/10 means capacity divided by 10. Most rechargable cells have a capacity rating in mAh or Ah. The batteries I have on hand are shown below. They are rated for 2200 mAh. So, if I keep the charging current below 2200/10 = 220 mA I don't need a charge controller chip. Since the clock should pull so little energy, this slow charge rate should fairly easily be able to keep up. So, whatever solar panel we select it needs to have a max output of 220 mA or less.

http://ecorenovator.org/forum/attach...1&d=1356626690



On to the voltage side of things. Max charge voltage should be about 1.41V per cell. The alarm clocks I've looked at both use 3 battery cells, so I'm looking for a max charge voltage of 3 x 1.41 = 4.23V.

Unfortunately, you can't just get a solar panel that puts out 4.23V. However, from what I've seen, 5V and 6V panels are readily available at the current levels we're talking about. So, we have to figure out a way to bleed off either .75V from a 5V panel, or 1.75V from a 6V panel.

To get this voltage drop is actually quite easy, and it is going to be done by another component we need for the solar setup anyway. This component is a diode. Its essentially a one way valve (check valve) for electronics. This prevents the solar panel from draining power from the batteries when there is no sun. A diode also has the added side effect of creating a voltage drop across it. It is similar to a plumbing system with a check valve in it. All check valves create a pressure drop across them because they cause a slight restriction. The same thing happens in an electrical circuit. There is always a voltage drop across a diode, and you can get diodes with different voltage drops.

For a 5V panel, this diode will drop the voltage by 0.75V bringing the total voltage down to a max of 4.25V.
BAT48 STMicroelectronics | 497-2512-1-ND | DigiKey

For a 6V panel, this diode will drop the voltage by 1.75V again bringing the voltage down to 4.25V
MUR1100ERLG ON Semiconductor | MUR1100ERLGOSCT-ND | DigiKey


So, I really just have to pick out a alarm clock, grab a solar panel off ebay, and then get a diode that will drop the voltage the correct amount. Really, its a pretty simple setup.

Alright, I went ahead and nabbed this solar panel:

New 5V 200mA 1W Monocrystalline Solar Panel Power Cell | eBay

Its rated for 5V and 200ma.

I also ordered the diode mentioned above for the 5V panel.

Just for kicks I decided to calculate the ROI on the 'solar investment' small as it may be. Thats not really the reason I'm doing this, but it I think the result is interesting.

First we have to figure out how much I spent on solarizing the clock. I'm not going to include the cost of the clock because I'd be buying a new one anyways. I'm also not going to include the rechargable battery cost because I'd be using these rechargable batteries anyways, just charging them with an AC powered battery charger. So, we really just have the cost of the solar panel and diode.

Solar panel: $7.99 shipped
Diode: $0.50 + $1.00 shipping

So, I have a solar investment of $9.49

http://ecorenovator.org/wp-content/u.../house-009.JPG

Now, lets find out how much that old clock is costing me. Using my killawatt, I know my old clock pulls a continuous 3W of power (which is rediculous for what it actually does). So, lets calculate the cost.

3W * 24 hrs/day = 72 watt hours per day = 0.072 kWh per day

I pay an average of $0.17 per kWh (fees included so this is actual cost).

0.072 * 0.17 = $0.01224 per day = $0.37 per month = $4.47 per year

Wow, that dumb old alarm clock costs me just over 1 cent a day to sit there and show me the time.

Last calculation:

$9.49 investment / $0.01224 savings per day = 775 day ROI

So there we have it, a 775 day ROI. That is just a bit over two years for the solar stuff to pay for itself. That is probably the best ROI I've ever seen for a solar PV product. I honestly expected it to be more than that.

http://ecorenovator.org/forum/attach...1&d=1357565131

I came back last night from a weekend away from home to see a package at my front door, and inside was the alarm clock.




http://ecorenovator.org/forum/attach...1&d=1357565147

So, of course I threw some batteries into it and started to tinker around with it. The battery compartment should be easily modified to add a few wires for the solar charger.



http://ecorenovator.org/forum/attach...1&d=1357565190

http://ecorenovator.org/forum/attach...1&d=1357565147

So far I'm liking it. The backlighting works very slick. As soon as the lights go out it turns the backlight on, and when the lights come on it turns off. The backlight is a bit brighter than it needs to be. It does illuminate the room a little. Its not as bright as the picture suggests, but it does let off more light than my old clock. I might try to tweak this if I can figure out where the power to the backlight is, but its not essential.

I'll be doing some more tinkering with it soon to get it ready to add the solar panel to it. I'd ideally like to be able to just add a charging port/plug to it like a laptop or cell phone has so it can easily be unplugged and looks nice. We'll have to see how much room there is inside though.

I also got the diode for the solar panel a day earlier. The solar panel is still on its way over from China.

So you plan to add an AC adapter so you can trickle charge it from the wall if needed?

Nope, I plan on just having a plug similar to that as a port to plug in the solar panel.

Depending on how power hungry the alarm clock is (I'll be testing this) will determine what I do. It might be easier to just plug the solar panel in for a charge up every month or so. 4 hrs of charging at 200 mah should get the batteries around a third charged, 8 hrs is 2/3rds charged. This is how my outdoor thermometer station works and I like it. If it is more power hungry than I anticipate, I'll just mount the solar panel on a window and run a wire across the room to the clock so its charging at every opportunity.

In any case, the plug just would make for a nicer setup.

Continuous trickle should extend battery life, no?

Yes, you are right. Less deep cycling will extend battery life. That is the way I originally thought of when I designed the charging system (solar panel hooked up 24/7). I probably will still do it that way.

Measuring the power draw will give more info on how flexible I can be with things.

Interesting. Going to hook it up to a digital multimeter for the DC current draw test?

Yeah, something like that. I'll pull the batteries out of the clock and wire them seperately. I'll have one multimeter measuring voltage and another meter measuring current. That should give me pretty precise power draw with and without the backlight on.

I got around to taking the clock apart and measuring power draw. The results were encouraging.



http://ecorenovator.org/forum/attach...1&d=1357942682

http://ecorenovator.org/forum/attach...1&d=1357942682

Using a 4.55V alkaline battery pack (nimh will be lower voltage), the clock uses a pretty impressively low 20 uA (microamps) with the back light off.



http://ecorenovator.org/forum/attach...1&d=1357942701

With the back light on, power consumption increase 11X to about 235uA! This isn't real surprising. The majority of the power consumption of even laptops is the back lighting for the screen.

However, even 235 microamps is still a very tiny draw. The batteries I'm using are rated for 2200 milliamp hours. That is the equiavalent of 2,200,000 microamp hours. So, even if the back light was on 24/7, the batteries should theoretically last 390 days! But, that is not the true case, in reality the current draw will be even lower, lets say 128 microamps (average of 20uA and 235uA). This almost doubles the battery life to 716 days. Still, this isn't realistic because nimh batteries are known to 'self discharge' on their own.

So, the solar panel must compensate for the draw of the clock plus the self discharging cells. I'll note you can buy low self discharge cells (aka precharged). Mine are not precharged cells, so I'll have to deal with this.

It was also mentioned on the blog post that my solar panel voltage may not be high enough to sufficiently charge the batteries. The commenter says you need at least 2 volts above the battery pack voltage to get a charging to happen. Testing will have to be done to ensure that the solar panel is in fact charging the batteries.

I reassembled the alarm.

Before doing so I soldered in some wires to the positive and negative battery leads and routed the wires out the back to hook up to the solar panel.

http://ecorenovator.org/forum/attach...1&d=1358089874


Tied the wires in a knot to stop them from being ripped out and/or putting stress on the soldered joints.

http://ecorenovator.org/forum/attach...1&d=1358089808


Drilled a hole in the back of the case for the wires to exit the case.

http://ecorenovator.org/forum/attach...1&d=1358089808


Then reassembled the clock.

http://ecorenovator.org/forum/attach...1&d=1358089808

http://ecorenovator.org/forum/attach...1&d=1358089808



Next up I'll have to find me some sunshine to test out weather the solar panel is actually going to charge up these batteries or not. Since the clock uses so little power it shouldn't take much to keep a sufficient charge.

You know you're an ecorenovator when... you put this much time into avoiding plugging your alarm clock into the wall. :D

Just kidding. Enjoying the project.

Haha, thats pretty true! It is a learning experience though, and a fun one. As you well know I quite enjoy this kind of tinkering.

I realized that I never finished writing up this little project. The last update was in 2013 and it has been working incredibly well ever since then. The nimh batteries plug away for months on end due to the awesomely low power consumption of the clock. One day in the sun literally powers this clock for months if not an entire year.

Here are some pictures of how I hooked up the 5V 200mah solar panel. The solar panel got a little pigtail and connector that I had kicking around. I forget what they're called, but its what computer fans use. I did not use a diode. I later wished I had gone with a higher voltage panel, so I did not add a diode as that would lower the voltage that the panel outputs. I simply plug the panel in when in use, and unplug it when its not in use.

https://ecorenovator.org/forum/attac...1&d=1564579415

https://ecorenovator.org/forum/attac...1&d=1564579432



Here is the full setup.

https://ecorenovator.org/forum/attac...1&d=1564579493

https://ecorenovator.org/forum/attac...1&d=1564579576